The present invention relates to a method of and an apparatus for low-pressure casting of a light metal alloy, and more particularly to a method of and an apparatus for low-pressure casting of a light metal alloy to produce a high-quality casting free of casting defects by introducing a molten metal alloy into a mold assembly constructed of materials of different thermal conductivities and causing the molten metal alloy filled in a mold cavity defined in the mold assembly to be progressively solidified in a certain direction due to the different thermal conductivities of the mold assembly materials, the method and the apparatus being capable of increasing the production efficiency by shortening the cycle time of the casting process.
Generally, the low-pressure casting process is widely employed in the mass-production of automotive parts or the like, for example. The low-pressure casting process produces a casting by pouring a molten metal alloy such as an aluminum alloy into a closed container, and exerting a low pressure to the surface of the molten metal alloy with a gas introduced under a relatively low pressure, typically compressed air, to force the molten metal alloy through a tube into a mold cavity defined in a mold assembly complementarily in shape to the casting to be produced.
One low-pressure casting apparatus generally used in the low-pressure casting process is schematically illustrated in FIG. 1 of the accompanying drawings by way of example.
The low-pressure casting apparatus, generally denoted at 2, basically comprises a mold assembly 3 and a molten metal feeder 4. The mold assembly 3 comprises an upper mold 5, a lower mold 6, and a pair of lateral molds 7a, 7b fitted in the upper and lower molds 5, 6, thus defining therebetween a mold cavity 8 complemetary in shape to a casting to be produced. The lower mold 6 is mounted on a fixed mold base 9, the lower mold 6 having a sprue 10. The upper mold 5 is mounted on a movable mold base 12 which is vertically displaceable by a pressure cylinder 11. The lateral molds 7a, 7b are coupled respectively to cylinders 13a, 13b, the lateral molds 7a, 7b being horizgntally movable for opening the mold assembly 3 by the cylinders 13a, 13b.
The molten metal feeder 4 includes a heating furnace 14 housing therein a pot 15 for storlng a molten metal, with a heater 16 disposed around the pot 15. A transfer tube 17 is partly placed in the pot 15 and has an upper end connected to the sprue 10. A tube 18 for supplying and discharging compressed air is connected to the heating furnace 14.
When compressed air under a prescribed pressure is supplied through the tube 18 into the heating furnace 14, the molten metal stored in the pot 15 is forced under the pressure applied to its upper surface through the transfer tube 17 and the sprue 10 into the mold cavity 8 in the mold assembly 3. After the molten metal has been filled in the mold cavity 8 and held therein under a prescribed pressure, the compressed air is discharged from within the heating furnace 14 via the tube 18, and the molten metal filled in the mold cavity 8 is cooled and solidified for a give period of time. A casting having a shape complementary to the mold cavity 8 is thus produced in one cycle (see FIG. 2) of such a casting process.
For producing a high-quality casting having a good structure free of casting defects in such low-pressure casting process, it is necessary to effect so-called directional solidification for allowing the molten metal in the mold cavity to be solidified quickly and progressively from an end of the mold cavity toward the sprue. The directional solidification is effective to remove casting defects such as shrinkage cavities which would otherwise be present in the casting.
Various methods have heretofore been proposed to control the temperature of the mold assembly used in the low-pressure casting process for directional solidification of molten metal filled in the mold cavity. The temperature of the mold assembly periodically varies in one cycle of the casting process as shown in FIG. 2. Since the mold assembly is generally made of steel or the like which has a low thermal conductivity, it is poor in its response to temperature regulation. Therefore, it is highly difficult to perform temperature control for directional solidification, and no substantially effective temperature control has been possible so far.
One conventional practice has been to adjust the casting cycle to increase the time in which to pressurize molten metal to be supplied to the mold cavity in order to keep the mold assembly temperature within a predetermined temperature range, and then to allow the molten metal to be solidified of its own accord over a relatively long period of time for thereby preventing casting defects from occurring. Actually, however, casting defects such as shrinkage cavities or the like tend to be developed in relatively thick portions of a casting. Another problem is that the casting efficiency is not increased since the cycle time is increased.